Deuterium and tritium—heavy isotopes of hydrogen—not only have numerous applications in science and medicine, but could also contribute to the energy mix of tomorrow as fuels for nuclear fusion. However, the process of filtering deuterium out of the natural isotopic mixture of hydrogen is at present both difficult and expensive.
Scientists from the Max Planck Institute for Intelligent Systems, the Max Planck Institute for Solid State Research, the University of Leipzig, Jacobs University Bremen, the University of Augsburg, and the U.S. Department of Energy’s Oak Ridge National Laboratory may be able to remedy this problem. They have presented a metal–organic framework compound that can be used to separate the two isotopes from normal hydrogen more efficiently than previous methods.
“Deuterium and, to a certain extent, tritium are useful in some applications,” said Michael Hirscher, who, as Leader of a Research Group at the Max Planck Institute for Intelligent Systems, played a key role in the research. “To date, however, it has been very difficult to separate deuterium from light hydrogen,” he says.
Deuterium is obtained from heavy water, which occurs in natural water at a concentration of just 15 parts per thousand. The heavy water is first isolated by a combination of chemical and physical methods, such as distillation, to obtain deuterium gas. The whole process is so intricate and energy-intensive that a gram of deuterium with a purity of 99.8 percent costs around $100, making hydrogen’s heavy brother around three times more precious than gold, although deuterium is more than 300 times more abundant in the oceans and Earth’s crust than gold.
“Our metal–organic framework compound should make it easier and less energy-intensive to isolate deuterium from the naturally occurring mixture of hydrogen isotopes,” said Dirk Volkmer, whose colleagues in the Department of Solid-State Chemistry at the University of Augsburg synthesized the material. In a metal-organic framework, or MOF for short, metal ions are linked by organic molecules to form a crystal with relatively large pores. Such substances are able to absorb large quantities of gas in relation to their weight.
As early as 2012, the scientists presented a metal-organic framework compound containing only zinc as the metallic component. It was able to filter out deuterium – but only at a temperature of minus 369 degrees Fahrenheit.
In the compound that this research team proposes, zinc and copper ions form the metallic nodes to serve as a deuterium and tritium filter.
The Augsburg-based chemists therefore replaced some of the zinc atoms with copper atoms, whose electron shells more selectively filter out deuterium and does so at higher temperatures. Michael Hirscher and his staff at the Max Planck Institute for Intelligent Systems and researchers at the Oak Ridge National Laboratory confirmed this property in various tests. Among other things, they determined the quantities of deuterium and normal hydrogen that the material absorbs from a mixture of equal parts of the two isotopes at various temperatures. They found that at minus 279 degrees Fahrenheit it stores 12 times more deuterium.
“At this temperature, the separating process can be cooled with liquid nitrogen, which makes it more cost-effective than methods that only work at minus 200 degrees [Celcius, or minus 328 degrees Fahrenheit],” said Hirscher.
The team of theoretical chemists headed by Thomas Heine, who recently assumed a chair at the University of Leipzig after previous teaching at Jacobs University in Bremen, helped interpret the collected data.
“Our calculations fitted the various parts of the experimental puzzle together into a coherent picture,” said Heine. “Our calculations for tritium would probably be right too. But this can only be experimentally confirmed under stringent safety procedures.”
Neutron scattering was one of the experimental tools used for this research. Neutrons are helpful in studying the motion of hydrogen because they are sensitive to light elements. Additionally, neutrons can distinguish among the signals coming from different isotopes like hydrogen and deuterium.
“In the metal organic framework, hydrogen molecules adsorb on different sites, by tracking the relative populations of hydrogen and deuterium in each site,” said Timmy Ramirez-Cuesta, Spectroscopy Group lead at ORNL’s Spallation Neutron Source. “Neutrons clearly elucidated the mechanism of isotopic separation.”